The Insulated-Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch and in newer devices is noted for combining high efficiency and fast switching. It switches electric power in many modern appliances such as: Variable-Frequency Drives (VFDs), electric cars, trains, variable speed refrigerators, lamp ballasts, air-conditioners, and even stereo systems with switching amplifiers.
IGBTs are often used for high voltage (e.g., greater than 600V) and high-current power converter applications. In these types of applications, a short-circuit of the load wire to a power source will result in a large current flowing through the IGBT, which is likely to damage the IGBT. Because of the potential for damage to IGBTs, gate drive circuits must detect IGBT short-circuit conditions and turn off the IGBT safely to prevent damage to the IGBT.
A common IGBT drive circuit 100 without short-circuit protection is shown in FIG. 1 where a load 112 is driven by current from IGBTs 108, which are in turn driven by gate drivers 104. The depicted circuit 100 is often referred to as a half-bridge circuit and is among the most important circuit configurations for power drives. The circuit 100 is shown to include two IGBTs 108 connected to one another at the circuit's 100 midpoint and the load 112 is connected to this midpoint. The midpoint corresponds to a circuit node where an emitter E of one IGBT 108 is connected to a collector C of another IGBT 108.
Problematically, as shown in FIG. 1, if the circuit 100 experiences a short (e.g., between Ground/common voltage and the circuit's 100 midpoint) then excessive current will flow through the top IGBT 108, most likely resulting in damage to the IGBT.
FIG. 2 depicts an illustrative circuit 200 that includes a short-circuit detection circuit 208 that enables the detection of a short-circuit event that could potentially damage the first IGBT Q1 in the circuit 200. It should be appreciated that the first IGBT Q1 may correspond to the same or similar component as the IGBT 108 depicted in FIG. 1. Similarly, the second IGBT Q2 depicted in circuit 200 may correspond to the same or similar component as the IGBT 108 depicted in FIG. 1.
As in FIG. 1, the circuit 200 is configured in a half-bridge configuration where a first driver 204a is driving the first IGBT Q1 and a second driver 204c is driving the second IGBT Q2. The short-circuit detection circuit 208 is shown to include a second driver 204b that senses the collector-to-emitter voltage Vce for the first IGBT Q1. An output of the second driver 204b provides information back to the first driver 204a such that if the short-circuit detection circuit 208 detects a short-circuit event. Most often, the short-circuit event that is detected by the short-circuit detection circuit 208 corresponds to a short between the midpoint 216 of the half-bridge circuit and ground. When such a condition is detected, the second driver 204b provides a signal to the first driver 204a that causes the first driver 204a to turn off. The second driver 204a is turned off in an effort to protect the first IGBT Q1 from overheating and/or damage due to increased current flowing from the collector C to the emitter E (known as collector-to-emitter current Ice).
During short-circuit protection test in circuit 200 for a short-circuit event, the third driver 204c is kept inactive, thereby keeping the bottom side of the second IGBT Q2 off. It should be appreciated that while circuit 200 is shown as including two IGBTs Q1, Q2, a circuit with a greater or lesser number of IGBTs may benefit from short-circuit detection and protection techniques.
FIG. 3 shows the current waveforms when a short-circuit event is detected by the short-circuit detection circuit 208 and the first driver 204a is subsequently turned off. In particular, during a short-circuit turn-off process, the first driver 204a initially turns on the first IGBT Q1 at time t0. At a point in time thereafter, the first IGBT Q1 enters into a short-circuit event and the collector-to-emitter current Ice rises rapidly. At time t1 the short-circuit detection circuit 208 detects the high collector-to-emitter voltage Vce and triggers a short-circuit shutdown. In response to the short-circuit shutdown, the gate-to-emitter voltage Vge begins to decrease. The rising collector-to-emitter voltage Vce generates a Miller current through the collector-to-gate capacitance Cgc of the first IGBT Q1. This Miller current injected into the gate G of the first IGBT Q1 generates a Miller plateau on the gate-to-emitter voltage Vge.
At time t2, the Miller current reduces as the Miller capacitance Cgc reduces rapidly with the higher reverse biasing of the gate-to-collector voltage Vgc. This, in turn, causes the gate-to-emitter voltage Vge to start decreasing again to turn off the first IGBT Q1. This results in the collector-to-emitter current Ice decreasing sharply with the quick fall in the gate-to-emitter voltage Vge. The sudden decrease in the collector-to-emitter current Ice and parasitic wire inductance Ls induces a spike in the collector-to-emitter voltage Vce, which ultimately causes the collector-to-emitter voltage Vce to reach a maximum value Vce_peak.
At time t3, the gate-to-emitter voltage Vge reaches the first IGBT's Q1 turn-off threshold, thereby reducing the collector-to-emitter current Ice to zero. This allows the collector-to-emitter voltage Vce to settle down to the terminal bus voltage Vbus and the turn-off process ends.
With the above in mind, circuit designers looking to utilize IGBTs have two primary concerns. First, the peak collector-to-emitter voltage Vce_peak should be less than the IGBT specified breakdown voltage (e.g., 650V). Second, the IGBT can only tolerate limited short circuit durations, which means that the duration of time between time t0 and time t3 should be less than a specified time for the IGBT (e.g., 10 us). To date many system trade-offs have to be made in order to simultaneously maintain the collector-to-emitter voltage Vce below a reasonable threshold while minimizing the short-circuit duration (e.g., the time between t0 and t3).